The present disclosure is directed to robotic wrists and more particularly to two-axis robotic wrists, otherwise commonly referred to as a pointing device, having a movable member such as a pointer that is movable in two axes relative to a base such as a robotic arm to which the wrist is mounted.
Robotic systems are commonly implemented to perform various manufacturing or other operations. Increasingly, such robots are required to traverse a complicated three-dimensional path at high speed with a high degree of accuracy. In these conditions, a robot having six degrees of freedom is needed to accomplish the desired motion. In practice, it may be desirable to separate these six degrees of freedom into three degrees of freedom of translation and three degrees of freedom of rotation, or orientation. The three degrees of freedom of orientation can be further divided into forearm rotation (e.g., rotation of a robotic arm) followed by articulation of a two-axis (degree of freedom) wrist carried by the forearm.
A universal joint, which is the simplest version of a two-axis wrist, exhibits locking at 90 degrees deflection and requires remote actuation away from the base (i.e., away from the robotic arm), both of which severely limit its applicability as a robotic wrist. To this end, two different classes of mechanisms comprised of the parallel combination of serial linkages have been implemented in an attempt to expand the operating capabilities of such robotic systems. One such class of mechanisms is a double universal joint (2UU) coupling that uses a parallel combination of double universal joints on common centers, where actuation takes place through the base joint of each UU chain, fixed at 90-degrees of shaft rotation relative to each other at both the base and the pointer ends. This class of mechanisms is undesirably overconstrained, which means that the component serial chains apply some of the same constraints against movement of the pointer.
One example of such a mechanism is disclosed by V. Milenkovic, “New Nonsingular Robot Wrist Design,” Robots 11 Conference Proceedings RI/SME, pp. 13.29-13.42, 1987, which has a central link separating the U-joint centers. This mechanism is able to achieve symmetric deflection of the outer gimbals in each double U-joint by internal drive mechanisms that enforce contra-rotation of the inner gimbals, operating on the separation between the U-joint centers and the fixed connections between the pair of double U-joint chains at their ends. Another example, disclosed by J. M. Wiitala and M. M. Stanisic, Design of an overconstrained and dexterous spherical wrist,” Journal of Mechanical Design, vol. 122, pp. 347-353, 2000, merges the inner gimbals to place each double U-joint on a common center. A spherical constraint linkage ties links connected to the inner gimbal to a floating equator ring common to the pair of double U-joints to accomplish the same goal.
The mechanism of V. Milenkovic results in a variable distance between the base and the pointer links with respect to their point of intersection on account of the offset between U-joint centers, whereas the mechanism of Wiitala and Stanisic is a pure spherical pointing device that maintains a constant distance from the sphere center. Such double U-joint pointing mechanisms limit each component U-joint to one-half the total deflection, moving the singularity to 180 degrees of deflection but are otherwise overconstrained, requiring precision manufacturing and offering substantial challenges in assembly of such systems.
Another class of mechanisms is referred to as a triple Clemens linkage (kinematic class 3RSR), which maintains a constant velocity relationship between input and output shafts (typically referred to as a constant velocity (CV) coupling), which are the base and pointer links in robotic systems, by using the parallel combination of three separate serial RSR Clemens linkages. This class of mechanisms is not overconstrained as are the 2UU mechanisms, but they require three actuator joints at the base, controlling pitch, yaw and distance of the base and pointer to their intersection center. In addition to having a redundant actuator (sometimes referred to as being overactuated because it has more actuators (3) than it has desired degrees of rotational movement (2) of the pointer), maintaining a constant distance of the pointer to the spherical center of rotation becomes a task of a computer control system.
There is a need, therefore, for a two-axis robotic wrist that is non-singular, non-overconstrained, and non-overactuated, and is more suitably also spherical, i.e., the pointer motion is spherical about its center point.